The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 199 22 391.2, filed on May 14, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to a backwater cycle of a paper machine that has at least one application device for applying a composition containing pigment and/or filler onto a fiber web that has not yet been completely dewatered in the wet zone. The backwater cycle includes a first backwater cycle that supplies a stock inlet with backwater, and a second backwater cycle that forms the remainder of the backwater cycle. The present invention further relates to a process for circulating water of a paper machine having a wet zone in which a composition containing pigment and/or filler is applied onto at least one surface of a fiber web being formed, and backwater is extracted from the fiber web being formed.
2. Discussion of Background Information
It is known from U.S. Pat. No. 5,152,872 to apply a pigment or filler onto a paper web that has not yet been completely dewatered, in order to produce thereby a multilayer paper whose outer layers have a pigment content considerably higher than that of the inner layer.
When such a pigment application or filler application technique is used in a conventional paper machine on a paper web that has not yet been completely dewatered, a problem arises that, after the paper machine has been running for a certain time, the wire cycle in which the stock inlet is included becomes enriched with filler or pigment. The result of such an enrichment is that the inner paper layer, which is initially low-filler, becomes likewise enriched with filler or pigments as the production process continues. This result runs counter to the desired distribution of the filler in the paper. In fact, an advantageous filler distribution in the paper is considered to be a state in which the filler or the pigments are essentially situated in the outer layers of the paper, while the inner layer is chiefly loaded with fiber components in order to achieve a high breaking strength. The chief advantage of such a steep filler distribution in the paper is the leveling of the surface structure that is decisive for the printing process, and homogenization thereof.
The present invention provides a backwater cycle of a paper machine and a process for circulating water of a paper machine that prevents the inner layer of the produced paper from being enriched with filler and/or pigment, even when the paper machine has been operated for a lengthy period.
During the paper production process, the amounts of water removed in a wire section of the paper machine are separated into two fractions, backwater I and backwater II. In modern paper machines, backwater I refers to the part of the backwater that accumulates on a forming roll, an upper wire suction box, a forming shoe, and/or other suction elements, in which the possibility of an upstream application of filler and/or pigment is not being taken into consideration. Backwater I is conducted in a short cycle without treatment and is utilized for thinning the slush of a machine vat immediately upstream of the stock inlet.
The water that accumulates at the wet suction elements, flat suction elements, and suction wire rolls are typically collected as backwater II. The clear water is then used for spray pipes and the like.
However, when filler and/or pigment is applied in the wire section of a paper machine, a portion of the applied filler and/or pigment can be removed by a subsequent vacuum suction element. Thus, re-using such water without intermediate treatment causes the thin pulp, which is initially distributed in a crosswise manner by the stock inlet and dewatered via a forming roll and wire suction box, to be enriched with filler and/or pigment. It is not possible to adjust a very steep filler distribution in the Z direction of the paper.
Therefore, in order to solve the above-described problem, the backwater cycle of a paper machine and the process for water circulation of the paper machine can be designed so that the water cycle in which the stock inlet of the paper machine is included is loaded from dewatering units that dewater the fiber web at a stage at which no composition containing pigment and/or filler has been applied onto the fiber web that is being formed. Each dewatering element that extracts moisture from the web subsequent to the application of filler and/or pigments can be consequently enriched with high pigment or filler components, so that this water would lead to a constant increase in the filler and/or pigment content of the layer formed by the stock inlet. Other units which would be undesirable to use for delivering backwater to the stock inlet include devices for catching cleaning liquid for wires which have come into contact with the pigment and/or filler application. This is because these wires are likewise charged with an increased filler and/or pigment content.
Thus, the present invention develops a backwater cycle of a paper machine that has at least one application device for applying a composition containing pigment and/or filler onto a fiber web that has not yet been completely dewatered in the wet zone. The backwater cycle includes a first backwater cycle that supplies a stock inlet with backwater and a second backwater cycle that accepts the remainder of the backwater. In this manner, the first backwater cycle is fed exclusively by dewatering elements that are arranged in the production process upstream, relative to a wire travel direction, of the application of a composition containing pigment and/or filler.
In this manner, the first backwater, which is returned to the stock inlet, is charged with lower or substantially lower filler concentrations or pigment concentrations than the pulp suspension that is fed to the stock inlet. This avoids the problem of the first backwater cycle, in which the stock inlet is included, being increasingly enriched with filler and/or pigment. Finally, in this way, a high portion of pigment and/or filler can be present in the outer layers of the produced paper web, while the fiber component maintains a constantly high value in the inner layer, and while the filler and/or pigment portion remains low.
The advantageous design of the backwater cycle of the present invention includes that the backwater of the second backwater cycle serves to feed the application of the composition containing pigment and/or filler. Since the second backwater cycle of the invention is fed at least chiefly by units that deliver water with a high pigment and/or filler content, this cycle is particularly well-suited to serve the water cycle for the applicator or applicators of the pigment and/or filler-containing composition.
It may furthermore be advantageous for the remaining backwater of the second backwater cycle, which is not being used to feed the application of the composition containing pigment and/or filler, to be processed with a depulping device, e.g., a microflotation device, to produce a clear filtrate. The clear filtrate formed from the second backwater cycle can be used later at any desired point in the production process of the paper web.
The presence of a depulping device, e.g., a microflotation device, is particularly advantageous when the backwater of the second backwater cycle is provided with a binder, e.g., starch, since this binder can then be carried out from the backwater by the depulping device and thereby avoid an enrichment of binder in the middle fiber layer of the finished paper.
Moreover, it is advantageous to introduce a disk decker upstream of a microflotation device, since a considerable portion of the remaining substances is removed from the backwater and the necessary filter area of the microflotation device can be reduced at the same water flow-through amount, thereby reducing costs.
It is also advantageous when a microflotation device and, optionally, a disk decker are present if the part of the backwater that is fed to the application of the composition containing pigment and/or filler is removed from the flotate of the microflotation device and/or from the overflow of the disk decker. This ensures that the portion of the pigment and/or filler-containing backwater that is provided with a higher pigment and/or filler content is fed to the application and, thus, leads to the maximum possible return of pigment and/or filler.
Moreover, according to the process of the instant invention for water circulation of a paper machine having a wet zone in which a composition containing pigment and/or filler is applied onto at least one surface of a fiber web being formed. Backwater is extracted from the fiber web being formed, so that the backwater that is not charged with pigments and/or fillers from the pigment and/or filler application is fed to the stock inlet.
Another result is that the inner fiber web of the finished paper is not enriched with filler and/or pigments and, in this manner, the breaking strength of the finished paper gains a constantly high value, even after lengthy production times.
An advantageous embodiment of the process provides that backwater charged with pigments and/or fillers from the pigment and/or filler application is fed to the application of the composition containing pigment and/or filler.
Moreover, it is advantageous to process the remaining backwater, which is not being used to feed the application of the composition containing pigment and/or filler and which is not fed to the stock inlet, with a depulping device, e.g., a microflotation device, to produce a clear filtrate and, optionally, to return it into the production process. The clear filtrate can be returned to the paper machine at any desired point.
If binder is present in the backwater of the second backwater cycle, the above explanations apply.
It is likewise advantageous to design the process so that a pre-cleaning by a disk decker takes place upstream of a microflotation device. This causes a reduction in the filter area needed in the microflotation device and, thus, a reduction in the cost of the process.
A further advantageous design of the process provides for the backwater fed to the application of the composition containing pigment and/or filler to be removed from the overflow of the microflotation device and/or the optionally present disk decker. The effect of this process design is that exclusively the high-content portion of the backwater is fed into the return to the application, and, thus, the amount of new pigment and/or filler needed can be reduced. Of course, the features of the invention described above and to be explained below can be used not only in the combinations provided, but also in other combinations or alone, without leaving the framework of the invention.
The present invention is directed to a backwater cycle of a paper machine having a wet zone. The backwater cycle includes at least one application device arranged to apply a composition containing at least one of pigment and filler onto a fiber web in the wet zone, a first backwater cycle that supplies a stock inlet with backwater removed from the production process upstream, relative to a web travel direction, of the at least one application device, and a second backwater cycle that accepts a remainder of the backwater.
In accordance with a feature of the invention, the at least one application device can be arranged to apply the composition to the fiber web while it is not completely dewatered. Dewatering devices can be provided so that the first backwater cycle can include first backwater removed exclusively by dewatering devices located upstream of the at least one application device.
According to another feature of the present invention, a first portion of the remainder backwater may be supplied to the at least one application device. Further, a depulper can be provided, so that a second portion of the remainder backwater can be produced in the depulper as a clear filtrate. The depulper can include a microflotation device. Still further, a disk decker can be arranged upstream of the depulping device. The depulper can be a microflotation device, and the first portion of the remainder backwater can include at least one of an overflow from the disk decker and flotate of the microflotation device.
In accordance with still another feature of the invention, upstream dewatering elements may be located upstream of the at least one application device. The first backwater cycle can include first backwater removed by the upstream dewatering elements. The first backwater can be removed exclusively by the upstream dewatering elements. Further, downstream dewatering elements may be located downstream of the at least one application device, such that the remaining backwater can include second backwater removed by the downstream dewatering elements. The second backwater can be removed exclusively by the downstream dewatering elements. Moreover, the paper machine may include at least one forming wire, and the backwater cycle can further include a cleaning liquid collector arranged to collect cleaning liquid applied to the at least one forming wire. The remaining backwater can include the collected cleaning liquid.
The invention is directed to a process for circulating water in a paper machine having a wet zone and at least one application device for applying a composition containing at least one of pigment and filler onto at least one surface of a fiber web being formed. The process includes extracting backwater from the fiber web, and feeding backwater, which is not charged with the at least one of pigments and fillers, to a stock inlet.
According to a feature of the invention, the process can also include exclusively feeding backwater, which is not charged with the at least one of pigments and fillers, to a stock inlet.
The process can also include feeding backwater, which is charged with the at least one of pigments and fillers, to the at least one application device. Further, the process can include exclusively feeding backwater, which is charged with the at least one of pigments and fillers, to the at least one application device. The backwater fed to the at least one application device can include cleaning liquid used for cleaning forming wires of the paper machine.
Moreover, the process may include processing a portion of the backwater, which is charged with at least one of pigments and fillers, in a depulping device to produce a clear filtrate. The depulping device can be a microflotation device. The process can also include preliminarily cleaning the portion of backwater in a disk decker located upstream, relative to a web travel direction of the microflotation device. Further, the process can include feeding backwater from at least one of the overflow of the disk decker and the flotate of the microflotation device to the at least one application device.
The present invention is also directed to an apparatus for forming a fiber web that includes at least one application device arranged to apply a composition containing at least one of pigment and filler onto a fiber web in the wet zone, at least one upstream dewatering device, located upstream, relative to a web travel direction, of the at least one application device, adapted to remove a first backwater from the fiber web, and a stock inlet coupled to the at least one upstream dewatering device, and adapted to receive the first backwater. At least one downstream dewatering device, located downstream of the at least one application device, is adapted to remove a second backwater charged with the at least one of pigment and filler, and at least a first portion of the second backwater is coupled to the at least one application device.
According to a feature of the invention, the apparatus can include a depulper, such that a second portion of the second backwater can be produced in the depulper produce as a clear filtrate. The depulper can be a microflotation device. Further, a disk decker can be arranged upstream of the depulping device. Still further, the first portion of the remainder backwater can include at least one of an overflow from the disk decker and flotate of the microflotation device.
The present invention is directed to a process for circulating water in a paper machine having a wet zone and at least one application device for applying a composition containing at least one of pigment and filler onto at least one surface of a fiber web being formed. The process can include extracting a first backwater which is not charged with the at least one of pigments and fillers, and feeding the extracted first backwater to a stock inlet.
According to yet another feature of the invention, the process can include extracting a second backwater which is charged with the at least one of pigments and fillers, and feeding a portion the extracted second backwater to the at least one application device. A remaining portion of the extracted second backwater can be a clear filtrate. The first backwater and the second backwater can be exclusive of each other.